1. Field of the Invention
The present invention relates to a Schottky barrier junction gate type field effect transistor, and more specifically to such a transistor having decreased parasitic resistance and capacitance.
2. Description of Related Art
In the prior art, of Schottky barrier junction gate type field effect transistors (called "MESFET" hereinafter), a GaAs (gallium-arsenide) MESFET having an active layer formed of an n-type GaAs crystalline layer has an excellent characteristics as a high frequency device, and GaAs MESFET elements typified by a high frequency amplification elements have been developed and supplied into commercial market.
Referring to FIG. 1, there is shown a diagrammatic sectional view of a typical conventional GaAs MESFET.
As shown in FIG. 1, the GaAs MESFET includes a semiinsulating GaAs substrate 11 having crystalline layers 12, 35, 36 and 37 formed on the substrate 11 by an epitaxial growth process in the named order. Specifically, the layer 12 is composed of an undoped GaAs layer having a thickness of 0.5 .mu.m, and the layer 35 is composed of an n-type GaAs layer having a thickness of 530 .ANG. and a carrier concentration of 5.times.10.sup.17 cm .sup.-3. The layer 36 is composed of an n-type GaAs layer having a thickness of 1000 .ANG. and a carrier concentration of 3.times.10.sup.16 cm.sup.-3, and the layer 37 is composed of a high concentration n-type GaAs layer having a thickness of 600 .ANG. and a carrier concentration of 1.times.10.sup.18 cm.sup.-3. A gate electrode 38, a source electrode 39 and a drain electrode 40 are formed as shown, and the gate electrode 38 is formed on the n-type GaAs layer 36 by etching the high concentration n-type GaAs layer 37.
The above mentioned construction of the GaAs MESFET has a large transconductance and a small gate bias dependency, since an n-type GaAs layer under the gate electrode is of a double-layer structure composed of the low carrier concentration layer 36 and the high carrier concentration layer 37. In addition, since the gate electrode is on the low carrier concentration layer 36, a gate break down voltage is large, and a capacitance of the Schottky junction formed between the gate electrode and the n-type GaAs layer.
Furthermore, since a GaAs region between the gate electrode region and each of the source electrode region and the drain electrode region is constituted of the high concentration n-type GaAs layer 37, the parasitic resistance has been reduced.
In the above mentioned conventional GaAs MESFET, however, the source electrode 39 and the drain electrode 40, which are ohmic contact electrodes, are not directly contacted to the n-type GaAs layer 35 forming an active layer. Namely, the source electrode 39 and the drain electrode 40 are contacted through the low concentration n-type GaAs layer 36 to the n-type GaAs layer 35, and therefore, reduction of the parasitic resistance has not been sufficient. In addition, since the gate electrode 38 is in physical contact with the high concentration n-type GaAs layer 37, a parasitic capacitance has been large.
Furthermore, in order to provide the gate electrode 38, it has been necessary to partially etch the high concentration n-type GaAs layer 37. However, since it is not possible to avoid dispersion in the etching process, MESFETs cannot have a stable and uniform characteristics.